Field of the Invention
The present invention relates to an anode for lithium batteries, a lithium battery comprising the anode and method for preparing an anode for lithium batteries.
Related Art
Recently, lithium batteries have high battery voltage and high energy density and use of lithium batteries is growing as a device to store or generate electric energy or as a main power supply for portable electronics, electric cars, and mobile communication apparatuses, etc.
A lithium battery is basically constituted by a cathode, an anode, an electrolyte and a separator, and is used as a primary battery or a secondary battery. For example, in a lithium secondary battery, lithium ions move from a cathode to an anode during charging, whereby it is possible to store an electrical charge, while lithium ions move in the opposite direction during discharging.
As an anode active material in a lithium battery, carbon materials such as graphite with which intercalation and release of lithium ions are possible are widely used practically, because carbon materials exhibit relatively high capacity and a good cycling characteristics. However, since downsizing and long time use are demanded for electronic devices recently, anode active materials are required to have higher capacity.
In such situations, as anode active materials for batteries, those comprising metallic lithium have been gathering attention. For instance, when metallic lithium is independently used as an anode active material, since an electrode potential is the lowest, an output potential of a battery combined with a cathode in which a common cathode active material is used becomes very large and, further, energy densities are also high, which is preferable (for instance refer to Patent Document 1). Therefore, a metallic foil of lithium is actually beginning to be used as an anode in lithium primary batteries.
However, in forming an anode by using a metallic foil of lithium, it is necessary to produce a metallic foil of several microns thick. It is difficult to produce such a metallic foil stably and inexpensively, and therefore productivity is poor, resulting in insufficient spread of practical applications.
In place of such metallic lithium foil, research has been undertaken with respect to a method wherein an anode is formed by depositing metallic lithium on an anode current collector by applying charge in a solution containing lithium ions. However, since the current density which can be applied in order to deposit metallic lithium on an anode current collector with a predetermined area has a limit, it naturally takes a long treatment time to deposit a required amount of lithium on a current collector so that metallic lithium functions as an anode. Therefore efficient production becomes difficult.
Further, in particular, in lithium ion secondary batteries, during charging and discharging, a defect which generates on the surface of metallic lithium sometimes acts as a core and results in the generation of thorn-like lithium called dendrite. Dendrite may sometimes grow in a way so as to penetrate the separator, which separates the cathode from the anode, causing fire, etc.
Here, Patent Document 2 discloses techniques relating to novel structures wherein carbon nanotubes that extend from a conductive composite material are anchored, as a structure for application as a current conductor and fully or part of an electrode for electrochemical power devices such as a conductive battery, supercapacitor, fuel cell, or the like. Further, Patent Document 2 discloses examples where the structure is applied as an electrode of, for instance, a lithium battery and indicates that the carbon nanotubes in the structure may comprise nanoparticles or silicon.
However, Patent Document 2 does not disclose lithium batteries in which metallic lithium is used as an anode active material or an idea of efficiently producing batteries which use metallic lithium as an anode material or an idea of suppressing the generation of dendrite during charging and discharging.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. H07-29602
Patent Document 2: Japanese Unexamined Patent Application (Translation of PCT Application), Publication No. 2012-532435